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This is the most persuasive iteration of your theory so far, for this layman.

It does raise several points for me. The floors might indeed have been 'shattered', but by all appearances they were actually pulverized, and I still cannot see how a natural collapse could give that result.

Yes, as you have stated before, additional energy in the form of explosives of some sort was required. And this additional energy also was sufficient to create a fairly symmetrical debris field, and to launch several large pieces of the exoskeleton/skin outwards about 400 feet.

I think I understand your use of the term "path of least resistance", but I'm not sure I agree with it. As another poster pointed out, it was actually the path of most resistance. Without the assistance of other 'additional energy' further down the structure, how can we end up with what is for all practical purposes, a free fall rate of natural collapse?

If it is indeed true that the buildings came down in the 10 second range. Which I admit we don't know that is a fact because me or you were not there to see and time it with our own eyes. But if that is the number. There would be some type a resistance on the way down. You are talking roughly 8 floors a second. Think about that. I'm not disputing any of the explosives or Thermite. I'm not disputing that something was going on high up in the towers. I'm just saying to get that to the ground in it's own footprint at that speed the resistance has to be addressed. Floor by floor we should have seen some change in speed of the collapse do to the mass under it. Remember the columns get much bigger and stronger when get to the lower 2/3rds of the building.So what your telling me Paul is that you would use the method of jumping on the elephants back to get it on it's belly?

The increasing strength of the columns as you go down has nothing to do with the floor collapse. The columns were larger at the base because they supported the columns above them. Floors were attached to the SIDE of the columns so each column which was 36' tall carried 3 floors and added the weight of 3 floors to the load it carried from above. Since each floor had the same strength and the same connection to the columns regardless of what floor it was, if the load was too large for THAT floor it would collapse and then the one below and so on.

The issue is how long this would take and it would only take about a second to bust through 10 floors or so. But this was definitely not a pancake collapse. The floor top floor was not "attacked" by falling debris evenly overloading the truss seats uniformly. Rather it was a chaotic assault compressed into a fraction of a second... like buck shot. The collapse floors became a chaotic avalanche which grew as it descended. It ground and pulverized everything which was not compressible or tearable into find grained debris and dust, The collapsing floors and their contents weighed north of 300,000 tons. It pressed against the facade from the inside pushing it away and it spilled into the core destroying the shaft walls and many of the lateral beams. None of the connections or components of the floor system could support or arrest the immense dynamic loads of the hundreds of thousands of tons of falling debris.

I would be with this all the way except your not explaning what brought the core down. What you explain is the floor sections that fasten to the core like you state. 10 floors a second? You can't drop a rock 80 feet in a second. How does something with floors bolted to steel drop faster than that? Now if you did blow the truss bolts like in Paul's video here. That would remove the resistance. But what about the core? The core size getting bigger doesn't effect the floor system like you said. But it does effect the cores strength.

This is at free fall with no resistance. So if the acceleration was 60% of free fall it would reach ~63 mph in about 3 seconds.

The columns collapsed from several causes and mechanisms. There were 47 and all of rows 500 and 600 survived the collapse of the floors as high 40, 50 and even 72 floors. Row 500 and 600 were still joined by the lateral beams up to about the 40th floor. A few of the columns stood with no bracing left. One of the topples over to the east. 501-601 wobbles back and forth. It was too tall slender and unstable for its height. Even without any explosives a 600' tall column made up of sections 36' long could not stand without guys and or lateral support. See Euler's formula about column buckling related to slenderness.

Most of the columns in the core cam apart with the avalanche which destroyed the lateral support between them and the debris falling chaotically broke them apart. The splices were not meant to resist that kind of movement and they were only welded at the edges of the splice plates.

The columns above the strike zone which likely were attacked by thermite or similar might have not only destroyed the splices, but destroyed the bearing so the columns buckled or translated laterally. The steel frame broke apart at the joints. This is seen throughout the debris. Have a look see.

My take away from Jon's video is that thermite could be used to weaken the steel. He doesn't say which steel, how many columns or lateral beams were attacked, but he basically trades on the correct assumption that if the structure is destroy it can no longer carry the loads and gravity will cause it to collapse.

This work completely supports or dovetails into the notion that once initiated from structural weakening or dissociating.. and partial unloading of enough columns so that the remaining ones are overloaded and buckle... the mass once supported will collapse driven by gravity.

The other way to destroy the building is to explode them completely... or attack every structural element. But we did not see that and there is no evidence of this in the debris.

You're a very all or nothing guy. You try to convince us that gravity alone was enough to the job, but the only other way to bring the buildings down would be to 'attack every structural element.' No CD requires attacking every structural element. I can't believe a real engineer would even make such an ridiculous claim. Especially, in regards to a building where they didn't even bother to weld the steel columns that made up the perimeter walls. I don't think it would be hard to blow those out, if you destroyed the cores with fuel air explosives. Wouldn't leave much for evidence either.

I hope people do not mis-understand SanderO. He is not saying explosives were not involved. He just questions the relative degree of explosives versus gravity. I think his thoughts are sound, but think explosives were relied on by the plotters more than he does.

I hope people do not mis-understand SanderO. He is not saying explosives were not involved. He just questions the relative degree of explosives versus gravity. I think his thoughts are sound, but think explosives were relied on by the plotters more than he does.

I personally don't believe that the perps would rely solely on the laws of physics to collapse the lower portion of the towers.What intrigues me is that the experiment in the video shows how exotic explosives, if properly harnessed, would massively reduce the need for "tonnes" of the stuff and would greatly reduce the visual effects one would expect to see from a proposed demolition job.The cores could very well have been rigged with these types of charges.

Do You recognizes that a commercial CD does not attack every structural connection. Most of the ones attacked are at the lower part and in the center to have the static loads cause the overloaded structure to collapse inward. The twins were very different from the strike zones down. The tops actually DO resemble a CD, but the bottom's destruction does not. No CDs have figured out a way to break some structural elements at the top and have the roof and a few floors take the whole building down.

What essentially happened with the twins is that the engineered destruction of the top section DID deliver the required overload which would crash through all the floors, strip off the facade and destroy the core so that it was absent enough lateral support to stand.

If the plane or explosive start was at the 105th floors the building would not have collapsed even if there was explosive CD on the top 5 floors. The released mass is less than the safety factor and there would be an arrest of the collapse. You'd be left with a tower with 5 floors of debris piled up on the 105th floor more or less.

Do You recognizes that a commercial CD does not attack every structural connection. Most of the ones attacked are at the lower part and in the center to have the static loads cause the overloaded structure to collapse inward. The twins were very different from the strike zones down. The tops actually DO resemble a CD, but the bottom's destruction does not. No CDs have figured out a way to break some structural elements at the top and have the roof and a few floors take the whole building down.

What essentially happened with the twins is that the engineered destruction of the top section DID deliver the required overload which would crash through all the floors, strip off the facade and destroy the core so that it was absent enough lateral support to stand.

If the plane or explosive start was at the 105th floors the building would not have collapsed even if there was explosive CD on the top 5 floors. The released mass is less than the safety factor and there would be an arrest of the collapse. You'd be left with a tower with 5 floors of debris piled up on the 105th floor more or less.

Oh really is that so is it? Taking the crazy heiwa approach to thing now are we? I wish someone could prove that the collapse could be arrestedbut that is never going to happen as we all know.

Not at all. The idea presented is that to cause a progressive collapse which does not arrest, you need to provide the suffient conditions for each floor to collapse. Since there is a safety factor of about 5, a single floors could support about 4 floors without it failing. This is very much like the local collapse of a few floors from the plane strike. That mass was insufficient to meet to minimum condition to fail the floor areas it descended on to this local collapse was arrested.

In both towers the minimum conditions were met and it applied to each floor below 92 for wtc 1 and below 77 for wtc 2. This explanation has nothing to do with small block crushing larger blocks as Bjorkmen and others try to reduce this to. That argument is true but not applicable. If you don't examine the event on a more micro level - that of structure and the composite floors in particular the are not looking at what failed.

Because of Willy's testimony I too believe that the lower regions were prepared for the imminent destruction.

Sander

Since the first time I saw the buildings coming down, it seemed that there was some sort of "cascading" dynamic at work there.

But it is true that in order to turn solids into dust, one needs both a hammer and a proper surface--a surface that will not give way. Because of the relative free fall collapse, there was no resistance at all. What happens if you swing a hammer against the air?

The floor collapse included all the collisions of the avalanche front with each successive floor. It was a chaotic highly energetic event where the material in the avalanche acted up itself much like the way a tumbler or rick crusher works.

There was an enormous roar when the collapse was taking place. That was the sound of millions of collisions or the materials in the building grinding, crushing, tearing apart and pulverizing everything in the towers. That sound was part of the energy released by the avalanche.

Falling concrete did not pulverize itself. Collisions of concrete with tens of thousands of tons of falling debris pulverized it.

If you take any floor in the lower section... say 52... Falling on this floor was the mass of 58 floors or about 150,000 tons, less the material which went outside the footprint and into the core. The facade "tried" to contained the avalanche but it was pushed away. You don't think even 2/3's or 100,000 tons crashing down on a 4" slab wouldn't pulverize it to pretty fine material? What do you expect from such an onslaught?

If you look at WTC 7 you will observe the same absence of concrete and the same sort of pulverization and we all saw it collapse DOWN with a destruction zone in the lower part of the tower. The falling mass of the top pulverized almost everything which was not crushable... created a huge dust cloud and left a small pile. We did not see that tower collapse from the top down. but we say a similar fate to the walls, concrete and contents - crushed and turned to dust. This is what one would expect from have such huge mass with so much kinetic energy.

The engineering in all three cases involved releasing the gravitational energy which would destroy everything which could not be crushed or torn to shreds - heavy steel etc.

For all practical purposes, the bottom two thirds of the tower offered the falling upper third the same resistance as air.

Quite incorrect. You are overlooking that the floors have a large inertia. The effect of a floor's resistance on the velocity of the falling debris is indeed minimal (due to the massive momentum of the falling debris), but the resistance of the individual rigid floors against acceleration (aka 'Inertia') is high because they have a high effective mass, and the acceleration will occur over a tiny fraction of a second (the collision is highly inelastic). The first debris to hit will be pulverised by what follows. Due to the chaotic nature of the collapse, these massive forces will be applied unevenly to the concrete slab resulting in further shattering and pulverisation of the slab.

Your rhetorical question, "What happens if you swing a hammer against the air?" is therefore based on a mischaracterisation of the system. The floor being hit might as well have been an anvil.

I came to a similar conclusion to SanderO on collapse propagation some time ago, although I put a lot less thought into the details The question in my mind since has not been, "How did the building end up as a pile of rubble?" (it seems to me inevitable, once a lot of mass starts to descend) but "Why did the building start to collapse?". And this is a good video that dispels some of the anti-thermite myths.

Due to the chaotic nature of the collapse, these massive forces will be applied unevenly to the concrete slab resulting in further shattering and pulverisation of the slab.

That's fine on an individual floor by floor basis, but what we saw of the collapses were far from "chaotic". If 7-8 floors were collapsing per second, you're suggesting that every eighth of a second there was a chaotic event which lead to the pulverization of each floor.I know it's a simplistic way of putting it but you get my point?

Welcome to the forum Trimble, but I do not find your post to be particularly persuasive.

Considering the nearly free fall rates of collapse, I see nothing whatsoever, including your random chaos theory, that would act as anvil to the hammer.

The concrete was not broke into smaller pieces, it was pulverized into a pyroclastic flow. An anvil of large proportions would have been required. The dust was so fine that it drifted for blocks in the air.

You can't pulverize concrete or stone for that matter without "collisions" as far as I know. So the process of turning concrete into granules and even dust would require grinding of some sort which is used in commercial rock crushing. You might want to read this:

which discusses how rock is crushed into smaller aggregates and powder. The concrete of the WTC floors was not especially strong, and had little or not stone aggregate and likely had something like light weight fly ash or crushed lava with lots of entrained air to make it light. This sort of concrete (no large aggregate) with little or no reinforcing was not structural and intended to resist tension. It was basically just a grout over the trusses and supported by the metal pans and would break up very easily.

The metal pans would have to be shredded to small pieces in the process and this is clearly less understood. But we've never see the kinds of forces that the floor avalanche contained before and how thin metal would respond to that.